So a local guy and fellow ham was looking for a 80 tube for his S-20R Sky Buddy. I went over to my mentor's place and dug out a NOS 80. When I put it on the tester, at first it tested fine. Then it went cloudy, something condensing on the inside of the tube and the reading on the tester dropped. Letting it cook for a while it cleared up and the reading came back up. However, I noticed a blue glow near the filament when pressing the test button, similar to how neon looks when excited with VHF RF.

The getters in tubes sometimes need to be reactivated after a tube has been in storage for a long time. Assuming your tube tester can handle the current, put the tube on the tester and leave it with the filaments lit for about half an hour. Do not press the "test" button, just let the filaments heat it up. If the blue glow in the tube is gone when you test it after that, it should be okay. But if you are still seeing the blue glow inside after the tube has had a chance to "bake," there may be more gas inside than the getter can deal with. I wouldn't use it in that case as it could flash over and wreck the power transformer in the radio.

_________________"Hell, there are no rules here--we're trying to accomplish something!"

The getters in tubes sometimes need to be reactivated after a tube has been in storage for a long time. Assuming your tube tester can handle the current, put the tube on the tester and leave it with the filaments lit for about half an hour. Do not press the "test" button, just let the filaments heat it up. If the blue glow in the tube is gone when you test it after that, it should be okay. But if you are still seeing the blue glow inside after the tube has had a chance to "bake," there may be more gas inside than the getter can deal with. I wouldn't use it in that case as it could flash over and wreck the power transformer in the radio.

OK, will do. The gettering material was scattered around on the side of the tube, not in a spot like you often see

Chris108 wrote:

I wouldn't use it in that case as it could flash over and wreck the power transformer in the radio.

If it was my radio I think I'd make a SS equivalent then. That would allow the transformer to run a little cooler. That would also mean a series resistor to drop the voltage, right? Take the dubious tube and bust the envelope an use it for a base.

80 has a voltage drop (loss) of about 60vdc. Solid state might have a few volts loss. Can the B+ handle an additional 50 or so volts? That's why there should be a voltage dropping resistor used with SS rectifier replacement.http://www.fourwater.com/files/fullrect.txt

The gettering material was scattered around on the side of the tube, not in a spot like you often see

This along with the description of the tube developing "fog" then clearing and having a blue glow inside the plate structures is exactly what one would expect from a mercury rectifier. The type 82 tube is the same size and has the same appearance as a type 80 tube except for the splotchy mercury deposits inside the glass rather than the more common getter flash. The blue glow inside the plate structure of a mercury rectifier is normal. The tube is almost certainly a good type 82 tube but it should not be used in place of a type 80 tube.

Norman

_________________"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems

Pulled the tube back out and with my glasses on (guess I have to admit i need 'em) the tube that went cloudy is an 83. I also have a 83V. Tube caddy shows them as a direct sub for the 80. Is this true?

The 83 tube requires a filament supply capable of providing 3 amps of current whereas the type 80 and 83V tubes require only 2 amps of current. Both the 83 and 83V tubes have much lower internal resistance than the type 80 tube and therefore will cause the radio to have higher B+ and run hot.

Norman

_________________"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems

Pulled the tube back out and with my glasses on (guess I have to admit i need 'em) the tube that went cloudy is an 83. I also have a 83V. Tube caddy shows them as a direct sub for the 80. Is this true?

No. In addition to lower voltage drop in the 83 and 83V, and the higher filament current required by the 83, the 83 is a mercury vapor rectifier. One of their characteristics is that the conduction turns on very sharply when the plate voltage becomes a few volts positive with respect to the cathode. This creates voltage spikes which may affect other circuitry unless small RF chokes are placed in series with each plate lead. Shielding the tubes was also sometimes necessary to prevent radiated noise or "hash" from affecting receiver circuitry. A radio designed for an 80 would probably not have those measures. 83V is a cathodic rectifier which means it needs more time to come up to operating temperature. This may be an advantage in some applications or a detriment in others.

But look at the bright side. 80 tubes are plentiful and inexpensive, particularly in ST or the later GT style style bulbs. 83 tubes, on the other hand, are scarce and in demand for tube testers.

_________________"Hell, there are no rules here--we're trying to accomplish something!"